1. Field of the Invention
The present invention generally relates to hydrogen storage, and more particularly to a system including hydrogen storage and hydrogen consuming devices, such as fuel cells.
2. Discussion of the Background
The use of fuel cells for power generation and transportation has the potential to provide an efficient and relatively environmentally benign alternative to conventional, combustion-based systems. Since most fuel cells operate on hydrogen, an import component of a fuel cell system is a hydrogen-storage unit to provide the fuel cell hydrogen on demand. Hydrogen is a gas under ambient conditions and has a small volumetric energy density, and as such hydrogen storage is problematic. Much development work has focused on providing hydrogen storage technologies with the capability to store of hydrogen at volumetric energy densities that exceed that of the gas at standard conditions. In addition, it is important that a hydrogen-storage device is economically viable, that it is lightweight, that it does not contaminate or otherwise interfere with fuel cell operation, and that it can be easily recharged.
One device for storing hydrogen is provided by tanks that can operate at pressures as high as 10,000 psi (69 MPa). One problem with tanks is that hydrogen is and can easily diffuse through metals, especially under high pressure, presenting flammability or explosion safety risks. Many tank materials cannot stand up to hydrogen diffusion at high pressures for a long period of time. In addition, high-pressure tanks are generally heavy and bulky, resulting in a large penalty, especially for transportation applications. Finally, hydrogen stored at high pressures presents safety risks associated with a catastrophic release of the compression energy of the gas.
Another device for stores hydrogen as a liquid at cryogenic temperatures. These devices are rather cumbersome and have very high energy penalties associated with liquefying the hydrogen. Because it is necessary to maintain the liquid hydrogen at a temperature of 20 K, there are efficiency and safety problems associated with the evaporation of hydrogen that occurs under normal conditions.
Yet another device for storing hydrogen includes materials that store hydrogen by sorption. One example of this type of material are hydrides, such as magnesium and magnesium-based alloys, that chemically bind with hydrogen as an interstitial compound, or hydrogen containing compounds such as complex hydrides and amide/imides. As an example, metal hydrides absorb hydrogen to form a solid that can release the hydrogen as a gas. Hydrogen can also be stored by physisorption of molecular hydrogen on high-surface area materials such as carbon and metal-organic-frameworks. In addition hydrogen can be stored in hydrogen containing compounds that are reacted with other compounds to release the hydrogen. An example of this type of material uses the reaction of NaBH4 with water. Although high pressures and extreme temperatures are not necessarily required for these materials to store hydrogen, many of the proposed hydrogen-storage materials systems are generally heavier than the hydrogen gas and liquid storage systems, and thus weight is an issue.
Thermal management is also an issue with fuel cells and with metal hydrides. Fuel cells usually operate best at greater than ambient temperatures and produce waste heat. Hydrides generate or require heat depending on how they are used. Thus, for example, the absorption of hydrogen by a metal hydride is normally exothermic, resulting in the heating of the hydride, and the desorption of hydrogen is normally endothermic, resulting in the cooling of the hydride. In addition, the absorption and desorption rates usually increase with temperature. From a safety perspective this behavior is desirable—hydrogen evolution from the hydride reduces the hydride temperature, and thus reduces the further desorption of hydrogen. When large amounts of hydrogen are required, however, heat must be supplied to the fuel storage unit to maintain high desorption rates. Another problem with metal hydrides and many other hydrogen-storage materials is that desorption typically requires about a sixth of the chemical energy available in the hydrogen, reducing the power available from a fuel cell-metal hydride system.
A new class of complex materials has recently been proposed for hydrogen storage. These materials, referred to herein as “decomposition-recombination materials,” or “D-R materials,” which release hydrogen by the decomposition into other compounds or elements with different stoichiometies of the non-hydrogen elements, and which will reversibly re-combine into the original compound upon hydrogen uptake. Thus, for example, alkali-earth-aluminum hydride (an “alanate”) such as NaAlH4 release hydrogen by decomposing into Na3AlH6, Al, and H2, and Mg2FeH6 releases hydrogen by decomposing into MgH2, Fe, and H2.
While D-R materials have the potential to store more hydrogen per unit volume with a smaller mass of material than classic metal hydrides, there are several difficulties involved with the use of D-R materials. First, as with metal hydrides, the uptake and evolution of hydrogen from a bed of D-R materials requires that the heat flow into the bed is sufficient to produce a sufficient flow of hydrogen, and that the heat flow out of the bed is sufficient to rapidly take up hydrogen upon charging. Second, D-R materials require temperatures higher that that of classic metal hydride fuel storage units, on the order of 80° C. or more. The elevated temperature requirement is the result of the kinetics of the reaction processes and possibly associated with the diffusion of decomposition components of the D-R materials. Thus, in the example of the alanates NaAlH4, some Al and/or Na species must diffuse over distances which are large on the atomic scale. This is in contrast, for example, with metal hydrides that include metal alloys that do not necessarily decompose. In solid/gas reactions the diffusion of reaction species is slow at room temperature and is more rapid as the temperature is increased. The use of beds of D-R materials for storing hydrogen is thus limited, in part, by thermal considerations.
Thermal management in prior art fuel cell systems using hydrides typically uses a heat transfer medium to provide the required heat transfer. Thus, for example, one prior art system circulates water between the fuel cell and fuel storage unit. This presents several problems. First pumping the water adds addition complexity, cost and energy penalties to the system. Second, water can be corrosive to the fuel cell and fuel storage unit housing, and is also highly reactive with some hydrides.
To realize the potential increased hydrogen storage capabilities in a fuel cell system, one or more of the following problems must be solved. First, the heat flow through a hydrogen-storage material should be sufficient to meet the storage and delivery requirement of the fuel cell-hydrogen storage system. Second, fuel cell and the hydrogen-storage material temperatures should be maintained at levels that provides for optimum performance. Third, the hydrogen-storage unit should be economical and require ancillary power or other needs that are less than those of current fuel storage units.